Proline-rich domains have been identified in several oral proteins, from salivary and bacterial sources. Proline-rich proteins, isolated from saliva, have been shown to bind Streptococcus sanguis to salivary coated hydroxyapatite. Proline-rich domains have also been isolated from the surface proteins of certain strains of S. sanguis. These domains have been shown to bind to and activate human platelets, which then aggregate into a thrombus. This research seeks to characterize the minimal interactive peptide of a proline-rich protein from the cell surface of S. sanguis which induces platelets to aggregate. To characterize the protein, a large scale isolation procedure will be developed to isolate the aggregation proteins in quantities greater than what are presently being obtained. The minimal peptide(s) from the aggregation protein will be isolated by high performance liquid chromatography, gel filtration and ion exchange chromatography after proteolysis with various enzymes. The amino acid sequence will be determined. Functional interactions with human platelets and immunological reactivity with existing poly- and monoclonal antibodies will be used to verify the isolation. Synthetic peptide(s), corresponding to the minimal peptide(s) will be produced and analyzed for crossreactivity with the minimal peptide(s). These peptides will then be used to study the structural requirements for interactions with human platelets. Nuclear magnetic resonance will be used to investigate the physical structure. Polyclonal and monoclonal antibodies will be used to investigate the immunological structure. Peptides, analogs to the minimal peptides, with selectively substituted amino acids, will be produced and analyzed for functional and immunological changes to determine what amino acids are absolutely required for interactions with human platelets. These data will help define the in vitro model for platelet- streptococcal interactions. Understanding the structural basis of these interactions may also lead to understanding the interactions of related proline-rich protein of salivary coated hydroxyapatite with Streptococcus sanguis.